Method for heating metal slabs or billets in continuous pusher-type furnaces

ABSTRACT

Metal ingots are heated in a continuous pusher-type furnace. In a heating zone the upper and lower surfaces of the ingots are heated differently so that in the initial stages of a soaking zone following the heating zone, the surface temperature of the upper surface of the ingots is above, and the surface temperature of the lower surface is below, the required ejection temperature of the ingots. In the soaking zone no heat is applied to the lower surface while some heat is applied to the upper surface and heat flow occurs through the center of the ingot from the upper to the lower surface.

United States Patent Pere et a1.

[54] METHOD FOR HEATING METAL SLABS OR BILLETS IN CONTINUOUS PUSHER-TYPE FURNACES [72] Inventors: Carlo Pere, 2 Via Mura del Prato; Fulvio Tornich, 18 Via Bottini, both of Genoa, Italy I [22] Filed: Nov. 10, 1970 [21] Appl.No.: 88,324.

[30] Foreign Application Priority Data Nov. 18, 1969 Italy ..7454 N69 [52] US. Cl. ..263/6 R, 263/52 [51] Int. Cl. ..F27b 9/14 [58] Field of Search ..263/6 R, 6 B, 52

[56] 9 References Cited UNITED STATES PATENTS 3,416,777 12/1968 Alexander, Jr ..263/6 R 3,342,468 9/1967 Sidwell ..'...263/6 B 1 July 18,1972

Primary Examiner-Charles J. Myhre Attorney-Herman, Davidson and Herman [57] ABSTRACT Metal ingots are heated in a continuous pusher-type furnace. In a heating zone the upper and lower surfaces of the ingots are heated'differently so that in the initial stages of a soaking zone following the heating zone, the surface temperature of the upper surface of the ingots is above, and the surface temperature of the lower surface is below, the required ejection temperature of the ingots. In the soaking zone no heat is applied to the lower surface while some heat is applied to the upper surface and heat flow occurs through the center of the ingot from the upper to the lower surface.

6 Claim, 5 Drawing Figures INVENTORS PERE,

TURN/CH,

I ATTORNEYJ.

Patented July 18, 1972 3 Sheets-Sheet 1 W A T C/N-FL 0 F01. u/o

BY ,dwm

METHOD FOR HEATING METAL SLABS OR BILLETS IN CONTINUOUS PUSIIER-TYPE FURNACES BACKGROUND OF THE INVENTION The invention relates to a method of heating in particular elongated ingots, such as slabs, billets or the like, in continuous furnaces, in particular pusher-type furnaces, in which the ingots passing through are first heated in a heating zone on the upper as well as the underside and then, in an adjoining soaking zone, on the upper side only.

The aim of the invention is to develop a method of this type which, as well as soaking the entire cross section of the ingot as uniformly as possible and achieving as closely as possible the required ejection temperature of the ingots, substantially reduces the heating time, that is to say the time spent by the ingots in the continuous furnace, and the specific heat consumption and produces a corresponding increase in the fur nace output.

SUMMARY OF THE INVENTION This aim is achieved in accordance with the invention by the fact that in the heating zone the upper side of the ingot and the under side are heated differently, in such a way that, at least in an initial section of the soaking zone, the surface temperature of the upper side of the ingot is higher than the required ejection temperature of the ingots. A heat flow develops from the hotter, upper side of the ingot, which continues to be heated, through the center of the ingot to the cooler, under side of the ingot, which is not heated any further.

It is preferable if, in a second section of the soaking zone the heating of the upper side of the ingot is also reduced or stopped.

In a preferred form of the method according to the invention, at the end of the heating zone the surface temperature of the underside of the ingot is close to the required ejection temperature of the ingots, whereas the surface temperature of the upper side of the ingot is already higher than the ejection temperature and higher than the surface temperature of the under side of the ingot.

In a further advantageous feature the surface temperature of the under side of the ingot is somewhat higher at the end of the heating zone than the required ejection temperature of the ingots, although it is always lower than the surface temperature of the upper side of the ingot.

In a further practical feature of the'invention the surface temperature of the upper side of the ingot is first raised still further above the required ejection temperature of the ingots by continuation of the heating in the first section of the soaking zone and is then maintained substantially constant.

The heat flow from the hotter, upper side of the ingot to the cooler, lower side which occurs in the soaking zone can be substantially stimulated, or accelerated, in that, in the first section of the soaking zone the surface temperature of the lower side of the ingot is first reduced, especially by a limited natural cooling of the under side of the ingot, to a value which is even further below the ejection temperature of the ingots and is then, by the absorption of heat from the upper side of the ingot, increased gradually up to the end of the second section of the soaking zone until it is at the ejection temperature of the ingots.

Thus, by means of a method according to the invention, ingots passing through are heated in the heating zone of a continuous furnace, particularly of the pusher-type, by means of both side heating, i.e. top and bottom heat, in such a way that the surface temperature of the under side of the ingot corresponds approximately to the required ejection temperature, e.g. the rolling temperature of the ingots, or preferably assumes a somewhat higher value than this ejection temperature, whereas the surface temperature of the upper side of the ingot is higher than the required ejection temperature of the ingots and higher than the surface temperature of the under side of the ingot which is reached simultaneously. The temperature of the center of the ingots is at this point lower than the temperatures at the surface of the top side of the ingots and of the under side of the ingots, i.e. an uneven temperature distribution develops in the cross section of the ingot. In the first section of the soaking zone the top side of the ingot is further heated so that its surface temperature always remains higher than the required ejection temperature of the ingots, or preferably it is heated further, strongly at first and then less strongly, after which it is maintained substantially constant up to the end of the first section of the soaking zone. The under side of the ingot, on the other hand, is not heated in the soaking zone and is even, preferably, allowed to cool down naturally, so that its surface temperature drops to a value below the ejection temperature. Consequently there is built up in the first section of the soaking zone a relatively marked temperature drop between the top side and the bottom side of the ingot, which gives rise to a corresponding thermal flux from the hotter, further heated upper side of the ingot, through the interior of the ingot towards the cooler, no longer heated lower side of the ingot. Consequently the temperature differences in the cross section of the ingot are equalized, i.e. the temperature at the center of the ingot rises further towards the required ejection temperature of the ingots, while at the same time the surface temperature of the lower side of the ingot also rises gradually and approaches the ejection temperature. In the second section of the soaking zone the heating of the top side of the ingot is also preferably diminished, or stopped completely. As a result the surface temperature of the top side of the ingot drops towards the ejection temperature, while at the same time the difference still persisting between the upper and lower surface temperatures and the center temperature are broken down and equalized. At the end of the soaking zone the ingots have a final temperature which is distributed practically uniformly over the entire cross section of the ingot and which corresponds to the required ejection temperature, e.g. the rolling temperature of the ingots.

The advantage obtained by the method according to the invention resides in the fact that, in consequence of the deliberately created and maintained temperature drop between the surface temperature of the top side of the ingot, which is higher than the ejection temperature, and the surface temperature of the lower side of the ingot, which in the soaking zone is lower than the ejection temperature, a strong thermal flow from above downwards through the center of the ingot is forced in the soaking zone, which ensures a particularly rapid and even soaking of the cross section of the ingot while achieving as closely as possible the required ejection temperature.

The method according to the invention can be carried out by means of various continuous or pusher-type furnaces, known in themselves. The pusher-type furnaces which have a heating zone with a system of slide rails and also top and bottom heating and an adjoining soaking zone with a ceramic soaking hearth and top heating, which are likewise known in themselves, are particularly suitable. A pusher-type furnace of this type which is especially suitable for carrying out the method, has a flat roof, parallel with the sliding plane of the ingots, and in this roof, both in the heating zone over the system of slide rails and also in the first section of the soaking zone at least, over the ceramic soaking hearth, there are provided preferably radiating burners which are distributed as uniformly as possible.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a pusher-type furnace, particularly suitable for the execution of the method according to the invention, in a diagrammatic, vertical longitudinal section,

FIG. 2 is a diagrammatic cross section of the pusher-type furnace along the line II-II of FIG. 1,

FIG. 3 shows temperature curves of the upper and lower ingot surfaces and of the center of the ingot, obtained on heating an ingot by the method according to the invention in the pushentype furnace shown in FIG. 1 and 2,

FIG. 4 shows the temperature distribution in the cross section of the ingot on passing through the heating zone of the pusher-type furnace, and

FIG. 5 shows the temperature distribution in the cross section of the ingot on passing through the soaking zone of the pusher-type furnace.

DESCRIPTION OF THE PREFERRED EMBODIMENT The pusher-type furnace illustrated in FIGS. 1 and 2 for heating ingots, especially elongated ones, M such as slabs, billets, or the like to a predetermined ejection temperature, for example a rolling temperature, has a two-zone construction which is known in itself and consists of a heating zone A with a furnace inlet 1 and an adjacent soaking zone B with a furnace outlet 2 and ejection flap 3. The direction of passage of the ingots M which lie transversely to the longitudinal direction of the furnace as shown by the arrow F.

The soaking zone B is constructed in the form of a ceramic soaking hearth 4. Disposed in the heating zOne A at the level of the sliding plane of the soaking hearth 4 is a cooled slide rail system, 5. The furnace roof 6 is flat and has a plane inside wall parallel to the sliding path of the ingots M. Heating takes place in the heating zone A both above and below the slide rail system 5 and in the soaking zone B only above the soaking hearth 4. For the underneath heating of the heating zone Z there are disposed on the front end below the ceramic soaking hearth several underfloor burners 8, distributed over the width of the pusher-type furnace. For the top heating, both of the heating zone A and also the soaking zone B, radiant burners 7 are provided in the plane furnace roof 6. These radiant top, or roof burners 7 are preferably distributed as uniformly as possible over the entire area of the furnace chamber.

The method according to the invention for heating the ingots M in the pusher-type furnace shown in FIG. 1 and 2 will now be described with reference to FIGS. 3 to 5. FIG. 3 shows the heating curves for an ingot M as it passes through the heating zone A and the adjacent soaking zone B of the pusher-type furnace. The temperature T is plotted in C on the ordinate axis and the abscissa axis corresponds to distance along the transit path S. The curve TO represents the course of the surface temperature of the upper side of the ingot in relation to movement of the ingot along the transit path S. The curve TU indicates the course of the surface temperature of the under side U of the ingot. The curve TK corresponds to the course of the center temperature of the ingot M. The required ejection temperature of the ingots is designated TE and in the example illustrated is I290 C.

In FIGS. 4 and 5 the temperature distribution in the cross section of the ingot M for the passage through the heating zone A (FIG. 4) and the adjacent soaking zone B (FIG. 5) is illustrated. The height H of the graphs corresponds to the thickness of the ingot M measured in the vertical direction between the upper side 0 of the ingot and the under side U of the ingot. The upper longitudinal axis of the graphs represents the surface temperature T0 in C of the upper side 0 of the ingot. The lower longitudinal side of the graphs corresponds to the surface temperature TU of the lower side U of the ingot. The individual successive curves indicate the course of the temperature in the cross section of the ingot after each meter of the transit path of the ingot M in the direction of the arrow F through the pusher-type furnace and they are correspondingly marked lm, 2m, 3m 31m, the assumption being that the length of the pusher-type furnace chamber is approximately 3 lm. In FIG. 5 the temperature scale is represented on a larger scale than in FIG. 4. Thus in practice FIG. 5 represents the prolongation, or completion of FIG. 4 on the right side, on a larger scale of lengths.

An ingot M entering the pusher-type furnace is heated, on its passage through the heating zone A on the slide rail system 5, on both sides, i.e. on the upper side by the radiant roof burners 7 and also on the under side by the underfloor burners 8. The ingot M is thereby heated in such a way that at the end of the heating zone A, on leaving the slide rail system 5, i.e. on running on to the soaking hearth 4, (that is approximately at the point S1 of the transit path abscissa S in FIG. 3) the surface temperature TU of the under side U of the ingot reaches a value TUl which is somewhat higher than the required ejection temperature TE. At the same time the surface temperature T0 of the upper side 0 of the ingot assumes a value TOI which is higher than the ejection temperature TE and than the surface temperature TU] of the under side U of the ingot which is reached simultaneously. The center temperature TK of the ingot M, on the other hand, reaches a value TKl which is considerably lower than the ejection temperature TE and than the surface temperatures T01 and TUl of the upper side 0 and the under side U of the ingot. Consequently an uneven temperature distribution develops in the cross section of the ingot which corresponds approximately with the curve marked 16.7m" in FIGS. 4 and 5, it being assumed that the heating zone has a length of l6.7m.

The soaking zone B is divided, from the operational i.e. heating aspect, into an initial, preferably longer section B1 and a second, preferably shorter section B2. In the first section B1 of the soaking zone B, the upper side of the ingot O is heated further by the roof burners 7 in such a way that its surface temperature TO, which is already higher than the ejection temperature TE, increases still further, at first markedly and then less so, and the reaches a maximum value T02 which is then maintained substantially constant as far as the end of the first section B1 of the soaking zone, i.e. approximately as far as the point S2 on the transit path abscissa of FIG. 3. The under side U of the ingot, on the other hand, is not heated in the soaking zone B and even undergoes a certain natural cooling on running on to the ceramic soaking hearth 4. As a result the surface temperature TU of the under side U of the ingot at first drops fairly quickly below the ejection temperature TE in the first section B1 of the soaking zone B and reaches its lowest value TUM for example approximately in the region of the temperature distribution curve marked 20m in FIG. 5. This produces a relatively marked temperature drop between the hotter, upper side of the ingot O and the cooler, under side of the ingot U as a consequence of which a corresponding flow of heat is forced from the upper side 0 of the ingot through the center of the ingot to the under side U of the ingot. This thermal flow effects, or accelerates the equalization of the temperature differences in the cross section of the ingot, whereby the center temperature TK rises further towards the ejection temperature TE and at the same time also the surface temperature TU of the under side of the ingot U is raised gradually and likewise approaches the ejection temperature TE. At the end S2 of the first section B1 of the soaking zone B the surface temperature T02 of the upper side of the ingot O is greater than the ejection temperature TE, whereas the surface temperature TU of the under side of the ingot U reaches a value TU2 which is still lower than the ejection temperature TE. The center temperature TK2 corresponds, however, to almost the required ejection temperature TE of the ingots. The associated course of temperature in the cross section of the ingot is represented by the curve 26m in FIG. 5.

In the second section 82 of the soaking zone B the heating of the upper side 0 of the ingot is also diminished or entirely discontinued. In consequence the surface temperature T0 of the upper side 0 of the ingot falls fairly rapidly towards the ejection temperature TE, whereas the surface temperature TU of the under side U of the ingot, as a result of the persistent thermal flow from above downwards, rises further towards the ejection temperature TE. Consequently the remaining difference between the surface temperatures TO and TU is reduced still more. At the ejection end 2 of the pusher-type furnace the surface temperatures TO and TU of the upper side 0 and the under side U of the ingots differ only to a slight, practically negligible amount from the required ejection temperature TE, while the center temperature TK corresponds to this ejection temperature TE, as can be seen from the curve marked 31m in FIG. 5.

Naturally the surface temperature T0 of the upper side of the ingot 0 must never exceed the maximum permissible heating temperature, e.g. the melting temperature of the surface of the ingot.

The invention is not restricted to the details of the example which has been illustrated and described. For example variations may be made in the values for temperature in the individual stages of the process or zones of the furnace and the form of construction of the continuous, or pusher-type furnace used for the carrying out of the method.

We claim:

1. A method of heating ingots in continuous furnaces including the steps of heating the upper and lower sides of an ingot in a heating zone of the furnace, moving the ingot to a soaking zone, heating the upper side only of the ingot in the soaking zone to produce an upper side surface temperature higher than the required ejection temperature of the ingots, simultaneously permitting the lower side surface temperature to cool to a temperature less than the required ejection temperature to develop a thermal flow from the hotter upper side to the cooler lower side through the center of the ingot.

2. A method as claimed in claim 1, including the step of reducing the heating of the upper side of the ingot following the step of heating the upper side only of the ingot,

3. A method as claimed in claim 2, including the step of maintaining the ingot in the soaking zone following the step of reducing the heating of the upper side until the lower side surface temperature is close to the required ejection temperature and the surface temperature of the upper side of the ingot is higher than an required ejection temperature and higher than the surface temperature of the lower side of the ingot.

4. A method as claimed in claim 3, including the step of maintaining the ingot in the soaking zone until the surface temperature of the lower side of the ingot is higher than the required ejection temperature of the ingots.

5. A method as claimed in claim 4, in which the step of heating the upper side only of the ingot in the soaking zone is maintained until the surface temperature of the upper side of the ingot first increases above the required ejection temperature and then is maintained substantially constant.

6. A method as claimed in claim 5, in which the step of heating the ingot upper side only in the soaking zone is maintained until the temperature of the surface of the lower side first falls to a point below the required ejection temperature of the ingots and then through absorption of heat from the upper side the surface temperature of the lower side gradually increases to the required ejection temperature. 

1. A method of heating ingots in continuous furnaces including the steps of heating the upper and lower sides of an ingot in a heating zone of the furnace, moving the ingot to a soaking zone, heating the upper side only of the ingot in the soaking zone to produce an upper side surface temperature higher than the required ejection temperature of the ingots, simultaneously permitting the lower side surface temperature to cool to a temperature less than the required ejection temperature to develop a thermal flow from the hotter upper side to the cooler lower side through the center of the ingot.
 2. A method as claimed in claim 1, including the step of reducing the heating of the upper side of the ingot following the step of heating the upper side only of the ingot.
 3. A method as claimed in claim 2, including the step of maintaining the ingot in the soaking zone following the step of reducing the heating of the upper side until the lower side surface temperature is close to the required ejection temperature and the surface temperature of the upper side of the ingot is higher than an required ejection temperature and higher than the surface temperature of the lower side of the ingot.
 4. A method as claimed in claim 3, including the step of maintaining the ingot in the soaking zone until the surface temperature of the lower side of the ingot is higher than the required ejection temperature of the ingots.
 5. A method as claimed in claim 4, in which the step of heating the upper side only of the ingot in the soaking zone is maintained until the surface temperature of the upper side of the ingot first increases above the required ejection temperature and then is maintained substantially constant.
 6. A method as claimed in claim 5, in which the step of heating the ingot upper side only in the soaking zone is maintained until the temperature of the surface of the lower side first falls to a point below the required ejection temperature of the ingots and then through absorption of heat from the upper side the surface temperature of the lower side gradually increases to the required ejection temperature. 